Known fluid flow controllers incorporate a needle valve movable between completely closed, and partially open positions, but if the needle valve is only open a small amount or is nearly closed, the resulting backlash or non-linearity affects the flow rate to, and at, the delivery outlet.
In some industrial processes materials condense out of a gas if the temperature falls below a certain value—the ‘dewpoint’.
An example is sulphuric acid, which condenses out of flue gas deriving from combustion of fuels containing sulphur. This can cause serious corrosion of the plant metalwork, and consequently it is important to know the dewpoint and to maintain the flue gas above this temperature.
In other processes materials (liquids or solids) may condense out of the gas and cause blockages in the process.
A dewpoint monitor is conventionally constructed by inserting into a flue gas for example a surface whose temperature is both controllable and measured. The surface temperature is initially set at a high value and slowly reduced. The surface temperature at the onset of condensation gives the dewpoint. The onset of condensation may be detected by various means, for example measuring the electrical conductivity between two electrodes embedded in the surface.
More sophisticated control algorithms may be used—for example controlling the temperature to maintain a fixed conductivity value. This maintains a fixed film thickness of the condensate on the surface. In this state the film is in equilibrium with the gas, i.e. the rate of condensation equals the rate of evaporation. The measured surface temperature then gives the equilibrium temperature, which is a true measure of dewpoint.
The dewpoint value may depend on the concentration of condensable material in the gas. Its measurement may therefore be used as a means to estimate this concentration. This has value in process control and pollution prevention.
More sophisticated algorithms may be used to estimate concentration (i.e. rather than a direct measurement of dewpoint). For example one may cycle the surface temperature above and below the dewpoint and record both the temperature cycle and the conductivity cycle. The relationship between these may differ depending on concentration. In simple terms, in the cooling part of the cycle a higher concentration gives a faster rate of deposition, which gives a thicker film, which then requires a higher temperature in the heating part of the cycle to remove the film and reduce the conductivity to zero, this ‘conductivity’ being taken as the indicator of film thickness, but other means such as optical means, could be employed to indicate film thickness.
In all the above there is a requirement for precise control of a surface temperature. This is conveniently achieved by means of a controllable air flow. The surface receives heat from the process (e.g. flue gas) and heat is removed by a cooling air flow. This cooling air flow must be very precisely controllable over a wide range of flow rates. The flow controller invention is of value in delivering cooling air to a dewpoint monitor.